The ability to control an iris opening of a camera lens plays an important role in image quality. An iris is used to maintain the optimal light level to the image sensor in the camera so that the image can be sharp, clear and correctly exposed with good contrast and resolution. The iris can also be used to control the depth of field in the image.
The iris opening can either be fixed or adjustable. The opening of a fixed iris cannot be adjusted, hence the name, and is fixed to a certain F-number. A fixed iris is often used in an indoor environment where the light level is more or less constant. The camera can compensate for changes in the light level by adjusting the exposure time and/or the gain of the image sensor.
An adjustable iris can either be manual or automatic. In a manual iris, the iris can be adjusted by manually turning a ring on the lens to either open or to close the iris. This is not convenient in an environment with changing light conditions such as outdoors.
There are two conventional types of automatic irises namely DC iris and video iris. Both types have a motor-driven automatically adjustable iris opening that responds to changes in the light level. Both types also use an analog signal (often an analog video signal) to control the iris opening. However, the main difference between the two types is where the circuitry for converting the analog signal into motor control signals is located. In a DC-iris lens, the circuit resides inside the camera, while in a video iris the circuitry resides inside the lens.
In situations with bright light, a camera with an automatic iris can be affected by diffraction and blurring if the iris opening becomes small. This problem is especially prominent in cameras with high resolution image sensors where the pixel size of the image sensor may be close to the diffraction limit of the camera optics. Therefore, the image quality in a camera is dependent on having an optimal iris opening for a given situation.
In order to optimize the image quality, a camera needs to have control over the iris opening inside the lens. The problem with conventional automatic iris lenses is that the control is not available to the camera or to the user of the camera. This is a severe disadvantage since it in this way makes it impossible to develop a control algorithm for controlling the optical performance of the camera. The depth of field, the resolution and the effect of optical aberration are also affected by not being able to control the absolute position of the iris opening.
The Precise-Iris lens, hereinafter referred to as P-Iris lens, is designed to address the shortcomings of an automatic iris lens. A P-Iris lens involves automatic and precise control of the iris opening (i.e. the diaphragm blades) with the aim of optimizing the image quality rather than controlling the light level. The P-Iris lens also provides improvements in contrast, clarity, resolution and depth of field, where objects at different distances from the camera are in focus simultaneously, all which are important parameters in the field of video recording, and especially in the field of video surveillance.
The P-Iris lenses iris is typically based on a stepping motor controlling the position of the diaphragm blades of the iris, and hence also the size of the iris opening. Each step of the stepping motor results in an increase or a decrease of the amount of incoming light reaching the image sensor. The amount of incoming light is a nonlinear function of the steps of the stepping motor, and is depending on the iris position. The steps, in F-number, are smaller when the iris is open and increases as the iris opening closes.
An important advantage of using a P-Iris lens having a stepping motor is that it may offer an absolute control of the position of the iris opening, which is not possible using a DC-iris. It is hence possible to set the iris to a specific aperture size by controlling the stepping motor. A motor driver is used to control the stepping motor in the lens. The basis for the control algorithm, run by the motor driver, is to maintain the iris opening in a predefined position for as long as possible under certain conditions. The predefined position, denoted position A, may for instance be based on the optimum position of the optical system or on a position set by the user defined by for example an F-number. The optimum is defined by the position where a high quality image in terms of parameters such as high contrast, low spherical aberration, low chromatic aberration, high resolution, low diffraction and a large depth of field, is achieved.
Apart from position A, a number of other iris positions matching certain light conditions may be predefined in an iris. For example a position B, corresponding to the diffraction limit of the system, and a position C, corresponding to a fully open iris position may be defined. Typically, the initial position for the iris at start-up is position A. If too much light reaches the image sensor the motor driver runs the stepping motor to iris position B, corresponding to the diffraction limit of the system. If not enough light reaches the image sensor, the motor driver runs the stepping motor to a fully open iris position (position C). At each position the integration time and/or the gain of the image sensor may be altered to optimize the image quality.
Together with the software, that is configured to optimize the performance of the lens and the image sensor, the P-Iris lens automatically provides the best iris position for optimal image quality in all light conditions. Thus, the P-Iris lens allows cameras to reach a new level of performance in image quality. The iris control will be especially beneficial for megapixel/HDTV cameras and demanding video surveillance applications.
To maintain the image quality during the transition time from one iris position to another it is important to let the stepping motor move the diaphragm blades in a pace adapted to give the exposure algorithm enough time to compensate for the changes in the incoming light using the gain and exposure time of the image sensor.
Different threshold values may determine when it is time to change the iris position. One threshold value could for example be defined as the brightness of the image at certain image sensor settings of the gain and exposure time. To prevent continuous movement of the stepping motor between two iris positions in a brightness region close to the threshold value, a hysteresis may be implemented in the control algorithm.
For some of the P-Iris lenses available on the market, the parameters needed for operating these lenses in an optimal way, such as the number of stepping motor steps to reach iris position A: optimum position, position B: diffraction limit (if too much light for optimum) and position C: fully open (when not enough light for optimum), have been measured by the manufacturer and hardcoded into the camera software. However, when a user wants to change the current lens into a P-Iris lens, for which there are no parameters coded into the camera software, the parameters must be made available to the camera in some way. One way of doing this is to manually enter the parameters, if provided by the manufacturer, into the camera. However, this inept way of manually having to make the parameters available to the camera can be quite tedious and time-consuming.